Continuous heating of steel strip



y 1957 J. M. FRANCK ETAL 2,799,605

CONTINUOUS HEATING OF STEEL STRIP Filed May 29, 1953 INVENTORS P C d J.

United States Patent '0 CONTINUOUS HEATING OF STEEL STRIP John M. Franck and Richard J. Stark, Baltimore, Md., assignors to Bethlehem Steel Company, a corporation of Pennsylvania Application May 29, 1953, Serial No. 358,492

4 Claims. (Cl. 148-16) This invention relates to improvements in the heating of metal, and particularly in the operation of a furnace for the continuous heating of steel strip.

It is an object of our invention to provide a method of operating a furnace so that continuous metal strip may be continuously heated therein without oxidizing the surface of the strip and without damaging or overheating the strip regardless of its gauge.

The foregoing and other objects of our invention will be more fully understood from the following description and claims together with the drawing which is a diagrammatic vertical section of a furnace adapted to the practice of our invention.

It is customary in the coating of metal strip with molten zinc or the like, to first clean the strip of oil, etc. and to continuously anneal the strip by passing it first through a heating furnace after which the annealing is completed in a non-oxidizing atmosphere and the strip is then Coated. It is necessary, in order to insure proper adherence of the coating metal, that the strip arrive at the coating bath with its surface clean and bright, that is, not oxidized.

In the continuous heating of metal strip in a furnace of the type here involved, it is obvious that the furnace must have a heating capacity capable of handling the heaviest gauge strip that it will be required to heat. On the other hand, such furnaces are called upon to heat strip of a wide range of thicknesses. It is obvious that when a light gauge strip is being heated, the furnace cannot be operated at its full heating capacity, as it would overheat and burn the strip.

The simplest way to reduce the heat in a furnace of the types described herein would be to turn off some of the burners. Thus, in the furnace shown in the drawing, in heating light gauge strip, the burners in the upper part of the furnace can be turned off, and the heat delivered by the lower panel of burners will be adequate to heat the strip to the desired temperature.

Such a method of operating the furnace, however, gives rise to certain difiiculties. With the volume of gases supplied to the furnace cut by a half or more, the pressure inside the furnace cannot be maintained at the desired point. This permits leakage of outside air into the furnace, with resultant oxidation of the surface of the strip. This condition is aggravated by the stack effect of the upper part of the furnace where the burners are shut off.

This condition could be corrected by supplying to the interior of the furnace, in the required amount, a manufactured gas of a non-oxidizing character. This, however, would be so costly as to be impractical.

We have discovered a means of overcoming the difficulties encountered in operating a furnace as described above, at practically no additional cost and with surprisingly beneficial results. Briefly, our invention consists in supplying to the non-operating burners in the upper part of the furnace, compressed air at atmospheric temperature. One result of blowing in air is that the internal furnace pressure is built up sufficiently to stop the leakage of air from the outside into the heating zone of the furnace.

Contrary to what might be expected, the blowing of air into the upper part of the furnace does not result in oxidizing the strip because, as a second result, this air cools the rising hot gases from the lower burners sulfioiently so that the incoming strip is not heated to a high enough temperature to permit it to oxidize. In the lower part of the furnace, where the strip is heated up to a relatively high temperature, conditions are maintained that will not cause the strip to oxidize.

Referring to the drawing, the furnace 10 is a vertical type heated by gas burners of the radiant refractory type. A plurality of these burners 11 are mounted in the walls of the furnace. For purposes of describing our invention, the furnace may be considered as having an upper zone 12 and a lower zone 13, each comprising approximately one-half the height of the furnace chamber 14. Flues 15 carry off the gases from the furnace to a stack. Outside the furnace at the top thereof are two perforated pipes 16, one on each side of the entering strip, through which compressed air is blown downwardly into the opening 17 through which the strip enters the furnace.

Continuous metal strip 18 from a coil, not shown, enters the furnace over roll 19 and travels vertically downward through the furnace and out of the furnace between rolls 20. Annealing of the strip is thereafter completed in a holding furnace, not shown, and the strip in its bright condition is then coated in a molten bath of coating metal.

In the operation of a furnace according to our invention, the lighted burners 11 in the lower zone 13 of the furnace are supplied with a combustible mixture of gas and air in predetermined pro-portions so calculated as to result in products of combustion which are non-oxidizing in character under the conditions of furnace operation. The unlighted burners in the upper zone 12 are supplied with plain air, blown in at atmospheric temperature.

The rising hot products of combustion from zone 13 are cooled by the air blown in through burners in zone 12 so that they are incapable of heating the strip, while in zone 12, to a temperature at which it would oxidize. The downwardly travelling strip 18 after traversing zone 12 enters zone 13 where it is cleaned and heated. The strip then continues on for further processing as above mentioned.

In actual operation we have found that by the use of our invention, a heating furnace is made capable of heating a complete range of thicknesses of strip without overheating or burning the strip or causing it to become oxidized. All gauges of strip have been found to present a bright appearance and to coat satisfactorily.

While we have described our invention in connection with a furnace divided int-o two approximately equal zones, it will be understood that the size of the zones relative to each other may be varied as desired, according to the thickness of the strip being heated. Satisfactory variations of the zone areas can be worked out on the basis of experience in the heating of strip according to our method. Also, the volume of air to be blown into the furnace through the upper burners can be determined in accordance with the actual operating conditions obtaining in any furnace installation.

We claim:

1. A method of operating a furnace for the continuous heating of uncoated metal strip comprising passing the strip vertically downward through the furnace chamber, heating the strip by supplying a combustible mixture of gas and air to the burners in a lower zone of the furnace chamber, and preventing oxidation of the surface of the strip by blowing air through a plurality of burners in an unheated upper zone of the furnace chamber in volume sufiicient to maintain the pressure within the furnace chamber at a point high enough to prevent leakage of outside air into said chamber.

2. A method of the character described comprising passing uncOated metal strip vertically downward through the furnace chamber, supplying heat to the strip in the lower Zone of said chamber and preventing oxidation of the surface of the strip by supplying cooling air to the strip in the unheated upper zone of said chamber in vo1- ume sufiicient to maintain the pressure with-in the furnace v10 chamber at a point high enough to prevent leakage of- 4 unheated upper zone of the furnace chamber in volume sufficient to cool the rising hot products of combustion from the lower Zone to a point where they cannot heat the incoming strip to a temperature high enough to cause it to oxidize.

4. A method according to claim 3 in which the volume of air is sufiicient to create an internal pressure in the furnace slightly in excess of the atmospheric pressure exteriorly of the furnace.

Elsey Mar. 29, 1932 Nachtman Jan. 18, 1949 

1. A METHOD OF OPERATING A FURNACE FOR THE CONTINUOUS HEATING OF UNCOATED METAL STRIP COMPRISING PASSING THE STRIP VERTICALLY DOWNWARD THROUGH THE FURNACE CHAMBER, HEATING THE STRIP BY SUPPLYING A COMBUSTIBLE MIXTURE OF GAS AND AIR TO THE BURNERS IN A LOWERZONE OF THE FURNACE CHAMBER, AND PREVENTING OXIDATION OF THE SURFACE OF THE STRIP BY BLOWING AIR THROUGH A PLURALITY OF BURNERS IN AN UNHEATED UPPER ZONE OF THE FURNACE CHAMBER IN VOLUME SUFFICIENT TO MAINTAIN THE PRESSURE WITHIN THE FURNACE CHAMBER AT A POINT HIGH ENOUGH TO PREVENT LEAKAGE OF OUTSIDE AIR INTO SAID CHAMBER. 